1. Field of the Invention
This invention is generally directed to apparatus and method for processing plural channels of related audio signals such as stereophonic, quadraphonic, et cetera. In particular, this invention is directed to apparatus and method for providing more accurately located psychoacoustic images when related (e.g., prerecorded) signals in such plural channels as simultaneously processed and transferred to plural corresponding acoustic signal sources by respectively corresponding electroacoustic transducers.
2. Description of the Prior Art
The general problem of faithfully recording (or transmitting) a naturally occurring field of acoustic signals and of faithfully reproducing an identically perceived field of such acoustic signals in another location is quite old in the art. There are a multitude of various stereophonic, quadraphonic and other sound reproduction systems which attempt with varying degrees of success to achieve such a desired result. However, as the continued proliferation of new and/or alternate sound reproduction systems continues, it is apparent that no perfect solution has yet been achieved.
In most simple two speaker versions of prior art reproduction systems, psychoacoustic image enhancement is usually accomplished in an attempt to place the outermost reproduced acoustic images beyond the actual physical locations of the left and right speakers. To achieve this enhancement, such circuits typically use either phase shift or phase inversion, variation in gain or combinations of both sometimes in concert with frequency tailoring, time delay, and/or compression or expansion. Some examples of these prior art image enhancement circuits may be seen by examining the following U.S. patents:
______________________________________ U.S. Pat. No. 3,246,081 Edwards (1966) U.S. Pat. No. 3,725,586 Iida (1973) U.S. Pat. No. 3,883,692 Tsurushima (1975) U.S. Pat. No. 3,911,220 Tsurushima (1975) U.S. Pat. No. 3,916,104 Anazawa et al (1975) U.S. Pat. No. 3,925,615 Nakano (1975) U.S. Pat. No. 4,027,101 DeFreitas et al (1977) U.S. Pat. No. 4,087,629 Atoji et al (1978) U.S. Pat. No. 4,087,631 Yamada et al (1978) U.S. Pat. No. 4,097,689 Yamada et al (1978) U.S. Pat. No. 4,149,036 Okamoto et al (1979) U.S. Pat. No. 4,192,969 Iwahara (1980) U.S. Pat. No. 4,209,665 Iwahara (1980) U.S. Pat. No. 2,218,585 Carver (1980) U.S. Pat. No. 4,219,696 Kogure et al (1980) U.S. Pat. No. 4,303,800 DeFreitas (1981) U.S. Pat. No. 4,309,570 Carver (1982) ______________________________________
With phase shifting or phase inversion, the stereo signal typically consists of a predominating channel signal appearing on one loudspeaker while the same signal appears in the opposite speaker but lower in amplitude and out-of-phase. Such circuits tend to create a "hole-in-the-middle" effect when the speaker is situated between the stereo speakers. That is, as the sound appears to come from further to the side of a listener, the sound tends to disappear from directly in front of the listener leaving a hole.
All prior art commercial systems have been criticized for creating poorly defined psychoacoustic images, weak center stage acoustic images (i.e., the "hole-in-the-middle" effect) or, especially in cases where expansion or compression functions are used, pyschoacoustic images which do not remain stationary. Furthermore, these commercial systems which provide cross-feeding do so over a broad range of frequencies.
A great deal of research has been performed with respect to the human hearing system and described in papers. For example, the acoustic intensity that can be barely heard at any particular frequency is known as the threshold of hearing or the threshold of audibility for that frequency. Threshold determinations fall roughly into two classes: "minimum audible field" (M.A.F.) and the "minimum audible pressure" (M.A.P.). The former is in terms of the intensity of the sound field in which the observer's head is placed; the latter, in terms of the pressure amplitude at the observer's eardrum.
The M.A.F. values directly relate to the usual mode of hearing, i.e., with the unaided ear. They are the more applicable when extended to include the effects of binaural hearing and of the listener's orientation with respect to the sound field. Such M.A.F. curves have been determined by Sivian and White, "On Minimum Audible Sound Fields", J. Acoust. Soc. Amer., 1933, 4, p. 288-321; Fletcher and Munson, "Loudness, Its Definition, Measurement and Calculation", J. Acoust. Soc. Amer., 1933, 5, p. 82-108, and others. The work of these researchers reveals that a pure tone having a given intensity level may be audible at some frequency levels but not audible at other frequency levels. Thus, the loudness of such a tone, as perceived by a listener, is therefore a function of not only its intensity but also its frequency.
Fletcher and Munson realized that the shape of these curves is determined, in part, by the direction around the head of the observer as he faces the source of sound. The external ear, or pinna, and the head have an effect on the sound. The head acts as a baffle and refractive object, and the cavities of the pinna have resonances which can be excited. Shaw, "National Research Council", Ottawa, Canada, Audio, Philadelphia, Pa., 1978, 5, p. 92-93 measured curves which show the effects of the external ear on the sound pressure at the eardrum compared with the free field when the sound source is placed at varying angles with respect to the ear.
The ability to localize the direction and to form some judgement of the distance from a sound source under ordinary conditions are matters of common experience. Two primary factors are employed to determine the horizontal arrival direction of a sound: (1) its relative intensity in the two ears of an observer and (2) its relative time of arrival at the two ears, or, for a sustained tone, the difference in phase between the waves arriving at the right and left ears. (See Kinsler and Frey "Fundamentals of Acoustics", John Wiley & Sons, Inc., N.Y. 1950, p. 370-392.) Thus, minimum audible field curves for binaural hearing were measured by Sivian and White, showing how the M.A.F. varies with the observer's azimuth relative to the wavefront.
With pure tones above 1400 Hertz, the ears cannot detect phase or time differences. See Zwislocki and Feldman, "Just Noticeable Differences in Dichotic Phase" J. Acoust. Soc. Amer., 28, p. 860, 1956. It has been found that it is only the intensity difference which furnishes a clue to directional hearing for these frequencies. The same intensity difference may arise at different azimuths at some frequencies, meaning that the intensity difference offers no exact clue to directional hearing.
With respect to depth localization, the Fletcher and Munson curves show that higher frequencies become more audible at greater intensity levels. Therefore, in the matter of distance between a sound source and an observer, a greater intensity level corresponds to a reduced proximity to the sound source. As a result of the relative increase in higher frequencies due to the Fletcher/Munson effect, the higher level also corresponds to an increased interaural sense, provided the sound intensity is equal in both ears. Thus, the Fletcher/Munson effect might be used to change the perceived distance of the sound, or to maintain the apparent distance of a sound which changes in intensity. See U.S. Pat. No. 4,204,092 to Bruney. Distance perception changes coincide with the changes in the lower and higher frequency ranges.
Despite this research that has been performed with respect to the human hearing system, no attempt has been made to apply findings for enhancing psychoacoustic imaging. Thus, the above-listed Carver patents teach the use of equalizers in the channels of a stereo circuit which have gains which resemble the Fletcher/Munson threshold curves. However, the gains have not been modified to account for the effect of the outer ear. Furthermore, the effect is applied to all signals independent of the degree of separation.